Fluked Burying Devices

ABSTRACT

A fluked burying device ( 1, 1 A), for augmenting the burying and holding capability of a drag embedment marine anchor and chain cable system while being fixedly located thereon, comprises a fluke member ( 8 ) inclinedly attached to a body member ( 7 ) including pockets ( 12 A,  12 B,  12 C) for accommodating links ( 16, 17 ) of a chain cable ( 4 ) passing there-through in a defined attitude such that axial and transverse loading is impressed in the chain cable ( 4 ) via separate links ( 17 A,  17 C) to maintain bending stresses therein similar to that occurring in a 5-pocket wildcat of a chain windlass and such that emergent links ( 16 A,  16 B) have sufficient articulation in service while embedded to permit the chain cable ( 4 ) to perform the function of and eliminate need for heavy expensive shackles. The fluked burying device ( 1, 1 A) enables soil loading from a plurality thereof to be distributed along a length of chain cable ( 4 ) and is preferably provided with a body member ( 7 ) less wide than the effective footing width of a chain cable ( 4 ) and preferably split about a plane of symmetry ( 5 ) in two halves ( 2, 3 ) for ease of transport within container gauge.

The present invention relates to cable-mounted fluked burying devicesfor increasing both the penetration and holding capacity of a dragembedment marine anchor and cable system.

Such fluked burying devices are disclosed in U.S. Pat. No. 3,685,479which shows a first fluked burying device for mounting on a wire hawserand a second fluked burying device for mounting in a chain cable to forman integral link thereof. The fluked burying devices interact withseabed soil to impress axial forces in the cable which increase holdingcapacity directly. The devices also increase holding capacity indirectlyby impressing transverse forces in the cable which counteractpenetration resistance thereof to promote deeper penetration of theanchor with consequent increase in capacity.

Disadvantages of the first fluked burying device include slippage ofclamping means attaching the device to the wire hawser and both handlingand operational damage arising from localised bending of the wire hawserdue to lack of articulation at both forward and aft ends of the device.

Disadvantages of the second fluked burying device include: a requirementfor structural adaptation to carry high transmitted loads in the anchorcable which are considerably in excess of those contributed by thedevice; a requirement for costly connecting shackles to provide anadequate degree of articulation while carrying high transmitted loads;and a Classification Society requirement for the application of a highproof load to the device and to the shackles, on completion ofmanufacturing, equal to that required for the associated chain cable.The last-mentioned Classification Society requirement applies to alldevices which are integral with the chain and through which chaintension is transmitted.

It is an object of the present invention to provide a fluked buryingdevice for increasing the penetration and holding capacity of a dragembedment marine anchor and chain cable system which overcomes theabove-noted disadvantages. It is a further object of the presentinvention to provide a modified fluked burying device capable of acting,additionally, as a substitute for a marine anchor used in a dragembedment marine anchor and chain cable system.

According to the present invention, a fluked burying device includes aplane of symmetry and comprises a body member and a fluke memberattached thereto at an acute angle of inclination to a longitudinal axisof said body member, and includes attachment means for holding said bodymember in a fixed position on a chain cable comprising a series oflinks, whereby said longitudinal axis is maintained substantiallyaligned with an axis of said chain cable, said body member being adaptedto maintain said chain cable extending in a defined attitude therein,said attachment means including first contact points on said body memberfor transferring axial load therefrom to corresponding points on a linkof said chain cable and second contact points on said body member fortransferring transverse load therefrom to corresponding points on a linkof said chain cable.

Preferably, said defined attitude comprises alternate links of saidchain cable being maintained in a plane at right angles to said plane ofsymmetry.

Preferably, said first contact points transfer axial load tocorresponding points on a first link of said chain cable and said secondcontact points transfer transverse load to corresponding points on asecond link of said chain cable.

Preferably, said second contact points are positioned such as toconstrain induction of bending stresses in said link of said chain cableduring operation of said fluked burying device to be similar to thatwhich occurs in a wildcat of a windlass for tensioning said chain cable.

Preferably, said second contact points are positioned on said bodymember at two locations to bear on two of said links spaced apart suchas to oppose turning moment induced in said fluked burying device duringinteraction with a seabed soil while penetrating therein with said chaincable taut.

Preferably, said two positions of said second contact points are spacedapart by not less than 12 times bar diameter of said links and,preferably, not less than 20 times said bar diameter.

Preferably, said second contact points are positioned such that saidcorresponding load transfer points on a link lying in a plane at rightangles to said plane of symmetry are spaced from said plane of symmetryby a distance in the range of 0.8 to 1.0 times the nominal diameter ofsaid link.

Preferably, said second contact points are positioned such that saidcorresponding load transfer points on a link lying in a plane at rightangles to said plane of symmetry are separated from a central point of acrown section, lying in said plane of symmetry, of said link by adistance in the range of 0.4 to 0.6 times the nominal diameter of saidlink measured in a direction parallel to said axis of said chain cable.

Preferably, said attachment means comprises a pocket in said body memberarranged of accommodate a link of said chain cable.

Preferably, said pocket constrains said link of said chain cable to liein a plane at right angles to said plane of symmetry.

Preferably, said fluked burying device is formed substantially in twohalves for opposed sideways assembly on said chain cable.

Preferably, said first contact points are located adjacent said flukemember whereby tensile loading impressed in said chain cable by saidfluked burying device substantially bypasses the portion of said bodymember lying forward of said first contact points.

Preferably, said body member comprises two elongate members eachdisposed substantially parallel to said plane of symmetry and eachextending along one of two opposed sides of said chain cable.

Preferably, said elongate members are plate-like.

Preferably, said pocket comprises an elongate slot perforating each ofsaid plate-like elongate members to accommodate said link.

Preferably, said elongate members are spaced apart by a distance between1.06 and 1.1 times the nominal diameter of said link.

Preferably, said body member has a width less than that of a link ofsaid chain cable, measured transverse to said chain cable.

Preferably, said elongate members each have a width less than a bardiameter of a link of said chain cable and, further preferably, lessthan 0.5 times said bar diameter, measured transverse to said chaincable.

Preferably, said elongate members are splayed apart at an extremity suchthat a link of said chain cable emergent from said extremity can swingfreely sideways from said plane of symmetry through an angle of up to20°, with 12° further preferred.

Preferably, said emergent link can swing in said plane of symmetrythrough an angle of up to 90° from said longitudinal axis.

Preferably, said fluked burying device including said second contactpoints at two spaced locations also includes a roll stabilizer.

Preferably, a fluked burying device terminating said chain cableincludes a yaw stabilizer.

Preferably, said fluked burying device is arranged such that a straightline containing a centre point of a forward crown section, lying in saidplane of symmetry, of a link in a foremost pocket in said body memberand a point of projection onto said plane of symmetry of a foremostpoint of said fluke member is inclined to said longitudinal axis at anangle in the range of 25° to 35°, with 30° further preferred.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings wherein:

FIG. 1 shows an oblique front view of a fluked burying device;

FIG. 2 shows an oblique rear view of the fluked burying device of FIG.1;

FIG. 3 shows a plan view of the fluked burying device of FIG. 1;

FIG. 3A shows a detail of FIG. 3 to a larger scale;

FIG. 4 shows a side view of the fluked burying device of FIG. 1;

FIG. 5 shows a view of the fluked burying device of FIG. 1 in directionX of FIG. 4:

FIG. 6 shows a section Y-Y of the fluked burying device of FIG. 4;

FIG. 6A shows effective footing width W of inclined studless chaincable;

FIG. 7 shows a side view of a modification of the fluked burying deviceof FIG. 1;

FIG. 8 shows fluked burying devices installed in a deep layer of soil;

FIG. 8A shows fluked burying devices installed in a shallow layer ofsoil over rock;

FIG. 9 shows an oblique view of the fluked burying device of FIG. 1 withroll stabilizers;

FIG. 10 shows an oblique view of the fluked burying device of FIG. 7with yaw stabilizers.

Referring to FIGS. 1 to 6, a fluked burying device 1 is formed in twoparts 2, 3 comprising a port half 2 and a starboard half 3, arranged foropposed attachment to each other about and in parallel with chain cable4. Fluked burying device 1 is symmetrical about a plane of symmetry 5(FIGS. 4 and 5) which contains axis 6 of chain cable 4 which extendsinternally within shank 7 thereof. Plane of symmetry 5 is verticallyorientated when fluked burying device 1 is buried in seabed soil 38 ofseabed 39 (FIG. 8). Port and starboard halves 2, 3 include shank 7 andfluke 8. Shank 7 comprises forward shank 7A, extending forward of fluke8, and aft shank 7B, extending aft of fluke 8. Shank 7 and fluke 8 areformed respectively by plates 9 and 10 joined together at junction 11.Plate 9 of shank 7 has three slots 12A, 12B, and 12C (FIGS. 1, 2 and 4)formed therein to function as pockets for accommodating links 17A, 17B,and 17C respectively of chain cable 4. Links 16 are held in plane ofsymmetry 5 whereas links 17 are held at right angles thereto. Slots 12A,12B, and 12C are axially aligned in a plane 13 (FIG. 4) disposed atright angles to plane of symmetry 5. Longitudinal axis 14 of flukedburying device 1 is defined by the intersection of plane of symmetry 5with plane 13 (FIGS. 3 and 4). Thus, slots 12A, 12B, and 12C hold axis 6substantially coincident with axis 14.

Port and starboard halves 2, 3 of fluked burying device 1 are assembledwith plates 9 in parallel with plane of symmetry 5 and withcorresponding slots 12A, 12B, and 12C registering with each other.Halves 2 and 3 and are fastened together sideways about chain cable 4 bymeans of bolts 15 passing through plates 9 and spacers 15A which serveto maintain sufficient clearance for links 16 (FIG. 5) of chain cable 4lying in plane of symmetry 5 to avoid clamping of links 16 by plates 9.Thus, spacers 15A space plates 9 apart by a distance in the range of1.05 to 1.1 times the nominal diameter D (FIG. 4) of chain cable 4.Slots 12A, 12B, and 12C provide a loose fit about corresponding links17A, 17B, and 17C to provide sufficient clearance in a directionparallel to axis 14 (FIG. 4) to allow chain cable 4 to stretch underextreme loading without being restrained by slots 12A, 12B, and 12C.

Plate 9 has lower extension 9A and upper extension 9B, at junction 11,provided to support fluke 8. Tapered plate ribs 18 are welded to plates9 and 10 to increase the bending resistance of fluke 8. Plate ribextensions 18A bearing against each other at plane of symmetry 5 takecompressive loading between halves 2 and 3 of fluked burying device 1(FIGS. 2 and 3). Distance E (FIG. 4) separating forward end 120 of slot12C from aft end 12E of slot 12B is chosen to provide minimal clearancebetween forward end 120 and link 17C and minimal clearance between aftend 12E and link 17B. Forward end 120 of slot 12C bearing on link 17Cconstitutes aft stop point 19A which prevents fluked burying device 1from being pushed aft on chain cable 4 when fluke 8 is subjected to soilloading during forwards embedment of fluked burying device 1. Aft end12E of slot 12B bearing on link 17B constitutes forward stop point 19Bto arrest fluked burying device 1 from being pushed forward on chaincable 4 when fluke 8 is subjected to soil loading during rearwardsrecovery of fluked burying device 1. Thus, stop points 19A and 19B acttogether to locate fluked burying device 1 in a fixed position axiallyon chain cable 4 while the before-mentioned allowance for stretchingensures that axial loading can be transferred only via stop points 19Aor 19B irrespective of the magnitude of tension in chain cable 4.

Since links 17A, 17B, and 17C are fitted loosely in respective slots12A, 12B, and 12C, relative movement between chain cable 4 and flukedburying device 1 causes either stop point 19A to bear on link 17C orstop point 19B to bear on link 17B to act as the sole point of transferof axial force into chain cable 4 from plate 9 of fluked burying device1 during forwards embedment and rearwards recovery respectively.Importantly, location of stop point 19A proximal to junction 11eliminates direct stress in almost the whole of forward shank 7A arisingfrom soil loading on fluke 8. Since tensile loading impressed in chaincable 4 aft of fluked burying device 1 bypasses both forward shank 7Aand aft shank 7B, no direct tensile stresses resulting therefrom areadditionally induced therein.

Transverse reaction bearing forces between shank 7 and links 17A and 17Cof taut chain cable 4 acting through bearing contact points A at theforward ends of slots 12A and bearing contact points B at the aft endsof slots 12C (FIG. 4) provide a resisting moment to counteract a momentarising from loading of fluke 8 which tends to rotate fluked buryingdevice 1 in plane of symmetry 5 relative to forward and aft portions ofaxis 6 of chain cable 4 external to fluked burying device 1. Bearingcontact points A and B are separated by a moment arm distance L (FIG. 4)approximately equal to 20 times the nominal bar diameter D (FIGS. 4 and6) of links 16 and 17 of chain cable 4 although, for a fluked buryingdevice 1 pocketing only two of links 17, distance L could be as low asapproximately 12 times diameter D. The bending moment induces bendingstresses in shank 7 which predominate in forward shank 7A (FIG. 4) whenconcentrated loading occurs at tip 20 of fluke 8 due to lodging, say, onan obstruction such as a rock. However, peak tensile bending momentstresses are not boosted by added direct stresses with the result thatshank 7 and, particularly, forward shank 7A may be of lighter and thuslower cost construction. This advantageous arrangement arises from theseparation distance M (FIG. 4), approximately equal to 15D, providedbetween the forward application points A of transverse loading and thepoints 19A of axial loading and is an important aspect of fluked buryingdevice 1.

Plate 10 of fluke 8 is inclined to plate 9 of shank 7 at an angle α(FIG. 5) in the range 90° to 115°, with 95° preferred. Junction 11between plate 10 of fluke 8 and plate 9 of shank 7 is inclined to axis14 at angle β (FIG. 4) which is in the range of 35° to 60°, with 50°preferred. Leading edge 21 of fluke 8 is inclined to junction 11 at anangle γ (FIG. 1) in the range 45° to 75°, with 60° preferred. A straightline 22 (FIG. 4), in plane of symmetry 5, joining centre 23 of a forwardcrown section 24 of pocketed link 17A to a point of projection 25 of tip20 of fluke 8 onto plane of symmetry 5, is inclined to axis 14 at anacute angle δ and forms, aft of tip 20, an acute angle ε with aprojection line 26 of junction 11 onto plane of symmetry 5. Angle δ isin the range of 25° to 35°, with 30° preferred and angle ε is in therange of 60° to 85°, with 80° preferred for fluked burying device 1 and65° preferred for terminal fluked burying device 1A (FIG. 7) describedbelow.

The length of junction 11 is approximately 17 times diameter D. Thelength of trailing edge 27 of fluke 8 is approximately 13 times diameterD. Thus, when diameter D equals 50 mm, the fluke area of fluked buryingdevice 1 projected on a plane (not shown), containing junction 11 and atright angles to plane of symmetry 5, is approximately 0.9 square metre.

Forward extremity 28 of plates 9 of forward shank 7A and rearwardextremity 29 of plates 9 of aft shank 7B each have a radius R (FIG. 4)made equal to 1.5 times diameter D. This allows forward emergent link16A (FIGS. 1,2, 4 and 5) to pivot on link 17A, which is restrained inslot 12A, through some 180° in plane of symmetry 5 and allows link 170(FIG. 4) to pivot simultaneously on link 16A through some 180° in aplane transverse to plane of symmetry 5. Similarly, rearward emergentlink 16B (FIGS. 2 and 4) can pivot on link 17C, which is restrained inslot 12C, but through only about 150° in plane of symmetry 5 due tochain cable 4 being restrained by making contact with fluke 8, whilelink 17E remains able to pivot simultaneously on link 16B through some180° in a plane transverse to plane of symmetry 5.

Transverse loading contact points A and B (FIGS. 3 and 3A) betweenplates 9 of shank 7 and links 17A and 17C are located at positions whichnormally occur when such chain links are loaded in a wildcat (also knownas a cable-lifter or gypsy) of a windlass. These load transfer pointsare known from FIG. 14 in published paper number 3813 of the OffshoreTechnology Conference, Houston, May, 1980, by A. Berg and A. Taraldsen,of Det Norske Veritas, which indicates that a central point of a loadtransfer area on a link lies at a distance of approximately 0.90 from atransverse plane containing the central axis of the chain link andseparated by a distance of approximately 0.50 from the centre of a crownsection of the link measured in a direction parallel to the central axisof the link. Accordingly, contact points A and B in shank 7 are arrangedto be spaced by distance X (FIG. 3A) from plane of symmetry 5 andlocated such as to be separated from central point 23 of crown section24 of link 17A or link 17C, respectively, by distance Y measured in adirection parallel to axis 6 of chain cable 4. Distance X is in therange 0.8D to 1.0D and distance Y is in the range 0.4D to 0.6D so thatlinks 17A and 17C are loaded as if in a wildcat.

In a 5-pocket wildcat, commonly adopted for service in the offshoredrilling industry, the angular displacement between adjacent supportedand unsupported chain links is 36°. When fluked burying device 1 isembedded in seabed soil 38, link 16A can become displaced angularly inplane of symmetry 5 through a maximum of 30° from adjacent supportedlink 17A due to coming into alignment with line 22 (FIG. 4) when alocalised force is applied at tip 20 of fluke 8 by, say, a rockyobstruction. Therefore, the transverse component of tension in chaincable 4 which gives rise to induction of bending stresses in supportedlink 17A is less than would occur in a wildcat by a factorsin30/sin36=0.85. Thus, constraint in slots 12A enables links 17A (and,similarly, 17C) to be operated at 15 per cent lower bending stress forequivalent axial tension than occurs in a wildcat.

Forward extremity 28 and rearward extremity 29 of plates 9 are splayedapart from lines A1 and B1 just aft and forward of points A and Brespectively (FIGS. 4 and 5) through a bend angle ⊖ (FIG. 3) of 20° toallow links 16A and 16B to pivot through angle ⊖ out of plane ofsymmetry 5 before being arrested by the splayed extremities 28 and 29.This allows azimuthal veering of chain cable 4 to be accommodatedwithout bending link 16A or link 16B. Since spacing 1.1D between plates9 is less than width 1.5D of a wildcat groove, the above-noted spacingof bearing points A and B from plane of symmetry 5 is maintained despiteextremities 28 and 29 being splayed apart.

The component of force acting transversely on fluked burying device 1 orterminal fluked burying device 1A when link 16A has pivoted 20° sidewaysout of plane of symmetry 5, due to azimuthal veering of chain cable 4while taut, is 34 per cent of tensile loading in chain cable 4. This issufficient to turn fluked burying device 1 or terminal fluked burialdevice IA into the direction of veering when embedded in seabed soil 38with the result that peak pivoting of link 16A, in actuality, isconsiderably less than 20°. Thus, link 16A (and, similarly, link 16B) issubjected only to tensile loading during such veering.

The above described combination of support, constraint, and freedom ofarticulation accorded links 170, 16A, and 17A and links 17C, 16B, and17E by plates 9 at extremities 28 and 29 enables chain cable 4 issuingfrom shank 7 of fluked burying device 1 to function normally, withoutincurring a penalty by way of increased bending stresses in links 17A or17C, and so fulfil the function of two heavy and expensive shackleswhich are thus rendered unnecessary.

The penetration resistance of chain cable 4 moving in direction P (FIGS.6A and 8), inclined at angle Ø to axis 6, in seabed soil 38, isproportional to area AP of unit length of chain cable 4 viewed obliquelyin direction P, where AP is gross area, neglecting internal apertures inchain cable 4 through which, in practice, soil is unable to flow. Thewidth W (FIG. 6A) of a unit length of rectangular strip footing of areaequal to AP is the effective footing width of chain cable 4 moving indirection P. W is a measure of the penetration resistance of chain cable4 in seabed soil 38. When viewed in direction P, links 17 appear toclose and merge together as Ø decreases, thus indicating that W variesinversely and non-linearly with Ø (FIG. 6A). With respect to studlesschain, in which width WL of link 17 is 3.35D, it may be determined thatfor 0°≦Ø≦10°, 3.35D≧W≧3.25D; for Ø=30°, W=2.69D; and for Ø=90°, W=2.48D.Thus, substantially maximum penetration resistance, being3.25/3.35×100=97 per cent, remains present when Ø is as high as 10° and,similarly, 80 per cent of maximum penetration resistance remains presentfor Ø equal to 30°. Accordingly, width WS (FIG. 6) of shank 7 isrestricted to being not greater than width WL, which is equal to 3.35Dfor studless chain, so that shank 7 does not add to the penetrationresistance of chain cable 4 at the critical stage of penetrating into afirm seabed surface 40 when Ø is small. Further, adoption of a lesserwidth for shank 7 in the range of 1.9D to 2.2D is preferred when Øincreases as high as 30°, which is known to occur when a series ofseveral fluked burying devices 1 penetrate deeply to depth Z (FIG. 8)below seabed surface 40.

Plates 9 and 10 are of thickness t9 (FIG. 6) and t10 (FIG. 5)respectively and may be of equal thickness for economy of fabrication.Thickness t9 of plates 9 is chosen to be less than diameter D so thatwidth WS of shank 7 is less than 3.35D. Preferably, thickness t9 ischosen to be not more than 0.60, and preferably less than 0.5D, tominimise the penetration resistance added by shank 7 to chain cable 4 athigher angles of inclination to axis 6 of the penetration direction P ofchain cable 4.

Referring now to FIG. 7, terminal fluked burying device 1A is amodification of fluked burying device 1 better suited for use as aterminal device at the end of chain cable 4. Shank 7 of terminal flukedburying device 1A is made hook-shaped in side view and lengthened toaccommodate five links 17 of chain cable 4 in five slots 12. Fluke 8 isenlarged and offset from plane 13 and axis 14 such that furthest aftpoint 30 of junction lilies on or below plane 13. Shank 7 of flukedburying device 1A is splayed apart adjacent forward extremity 28A in thesame manner as for fluked burying device 1. Also, links 16A and 17A aresupported and loaded in the same manner as for fluked burying device 1.The increased offset loading of fluke 8 results in higher bendingmoments in shank 7. These induce higher stresses which are accommodatedby increasing appropriately the section depth of plates 9 with distancefrom end 28A of shank 7 and adding doubler plates 31 between plates 9 toact as spacers which increase the strength of shank 7 without increasingwidth WS thereof. Two contoured spacer plates 32 are welded to each ofplates 9 to act both as spacers and provide stop points 19C and 190which differ from previously shown stop points 19A and 19B in thatpenultimate link 33 and ultimate link 34 of chain cable 4, lying inplane of symmetry 5, are restrained thereby. Heavy bolts 35 pass throughplates 9 and spacer plates 32 to prevent spacer plates 32 from beingforced apart by large forces transferred thereto from ultimate link 34.Ultimate link 34 may also serve as a lug for attachment of a pendantline to facilitate installation. Plates 9 of shank 7 are extended to tip20 of fluke 8 by way of tapered stiffeners 36 and 37 to enable terminalfluked burying device 1A to withstand high concentrated loading appliedat tip 20. Thus, terminal fluked burying device 1A is, in essence, amarine drag embedment anchor constructed in two halves for assemblysideways onto chain cable 4 with chain cable 4 therein acting as aparallel load-bearing element and providing sufficient articulation toeliminate both a need for a conventional heavy and expensive shackle andthe high penetration resistance penalty associated with such a shackle.

Fluked burying devices 1 and 1A may be fitted with roll stabilizers 42and yaw stabilizers 43 respectively (FIGS. 9 and 10).

Roll stabilizers 42 (FIG. 9) comprise rectangular plates 43 attached oneat each side of fluked burying device 1 to edge 44 of fluke plate 10 ata position approximately midway along the length of edge 44 with edge 45of plate 43 being aligned with edge 44. Plates 43 lie in plane 46 whichis disposed at right angles to plane of symmetry 5 (FIGS. 3 and 5) andwhich is inclined at an angle Δ to axis 14 of fluked burying device 1.Plane 46 intersects plane of symmetry 5 in line 46A. Angle Δ issubtended by line 46A and axis 14 and is in the range of 0° to 40°, with20° preferred. The area of plate 43 is in the range of 8 to 12 per centof the area of each plate 10 with 10 per cent preferred. Soil incidenton plate 43 produces a force parallel to plane of symmetry 5 which givesrise to a roll moment about axis 6 of chain cable 4. Any rolling actionof fluked burying device 1 causes one of plates 43 to bury deeper insoil 38 than the other and so gives rise to a net imbalance in rollmoments about axis 6 acting in opposition to the rolling action, thusproviding a roll stabilizing effect. Fluked burying device 1 need bestabilized only in roll since tension in chain cable 4 resists yawmisalignment, between axis 14 of fluked burying device 1 and thoseportions of axis 6 of chain cable 4 adjacent and external to flukedburying device 1, by giving rise to a large countervailing moment.

Yaw stabilizers 46 (FIG. 10) comprise substantially triangular plates 47attached one at each side of terminal fluked burying device 1A. Aforward apex 47A of triangular plate 47 is attached to edge 48 of flukeplate 10 at a position approximately midway along the length of edge 48.Upper edge 49 of triangular plate 47 lies in plane 50 containing flukeplate 10 which is extended locally to support triangular plate 47.Triangular plate 47 is located in plane 51 which is inclined at angle Ωto plane of symmetry 5 (FIGS. 3 and 5) such that the intersection (notshown) between plane 51 and plane of symmetry 5 is at right angles toaxis 14 of terminal fluked burying device 1A. Thus, when angle α (FIG.5) of fluked burying device 1A equals 90°, angle Ω is included betweenedges 48 and 49. Angle Ω is in the range of 10° to 35°, with 20°preferred. The area of triangular plate 47 is in the range of 8 per centto 20 per cent of the area of each fluke plate 10, with 14 per centpreferred. Any rolling of terminal fluked burying device 1A causes yawto occur due to a lack of a countervailing moment being produced fromthrough tension in chain cable 4, as previously mentioned for flukedbuying device 1. Thus, terminal fluked burying device 1A is subject to aroll-yaw couple. Since the resistance in soil 38 to rolling of terminalfluked buying device 1A is greater than the resistance to yawing, it iseasier and more effective to stabilize in yaw than in roll.Stabilization in yaw preventing fluke point 20 of fluked burying device1A from moving sideways along a helical roll-yaw path that wouldotherwise occur under the influence of a roll-yaw couple. Any rollingaction of fluked burying device 1 causes one of triangular plates 47 tobury deeper in soil 38 than the other and so gives rise to a stabilizingnet imbalance in yaw moments about the articulation contact pointbetween links 16A and 17A of chain cable 4 adjacent foremost end 28A ofshank 7. This acts against incipient rolling action by preventing flukepoint 20 from moving sideways along the unstable helical roll-yaw path.

Several fluked burying devices 1 may be used in conjunction withterminal fluked burying device 1A (FIG. 8) to enable the full loadcarrying capability of any size of chain cable to be exploited fully.Each fluked burying device 1 effectively cancels adjacent penetrationresistance of chain cable 4 in seabed soil 38 to allow terminal flukedburying device 1A to achieve penetration depth Z below seabed surface 40sufficient for the load contributions of each device, in aggregate, tomatch the breaking load of chain cable 4. In seabeds 39 havingpenetrable soils 38 of limited vertical extent overlying impenetrablerock layer 41 (FIG. 8A), an extended series of fluked burying devices 1may be deployed and installed against layer 41, again with the loadcontributions from each device, in aggregate, being able to match thebreaking load of chain cable 4.

Terminal fluked burial device 1A, now simply referred to as flukedburial device 1A, may also be used in place of fluked burying device 1to exploit the advantage of having a fluke offset from axis 6 of chaincable 4. A series of fluked burying devices 1A may, for example, be usedon a hard sea bed surface 40 where offset flukes 8 are able to penetratealmost fully before underside 41 (FIG. 7) of shank 7 bears on surface 40of seabed 39 to resist or even arrest further embedment.

It will be readily appreciated that variations of the above describedfluked burying devices are possible within the scope of the presentinvention. For example, plates 9 of shank 7 may be provided with adifferent number of slots 12 than shown in the accompanying drawings.Also, additional elongated split spacers (not shown) between plates 9,split about plane of symmetry 5, may be provided along the periphery ofshank 7 to enable external welding along the split line to be performed,after assembly of fluked burying device 1 onto chain cable 4, if sodesired for long term service.

The present invention provides numerous advantages. Since fluked buryingdevices 1 and 1A are each constructed in two halves for final assemblyon chain cable 4, the number deployed and the spacing between devicescan be selected to suit soil conditions and user preferences. Thedevices are readily and cheaply transportable in gauge in standardshipping containers while disassembled. Incorporating chain cable 4within fluked burial devices 1 and 1A and the splaying of plates 9 ofshank 7 to provide an adequate degree of articulation eliminates needfor expensive shackles. The use of chain cable 4 as a major load-bearingelement within devices 1 and 1A allows a significant reduction instresses to be achieved which results in reduced structural cost. Theability to distribute loading along a length of chain cable 4 by using amultiplicity of fluked burying devices 1 in conjunction with a terminalfluked burying device 1A allows high holding capacity to be obtainedfrom seabeds having shallow or deep sediment conditions in a manner andat a low cost hitherto unobtainable.

1. A fluked burying device (1, 1A) includes a plane of symmetry (5) and comprises: a body member (7) and a fluke member (8) attached thereto at an acute angle of inclination to a longitudinal axis (14) of said body member (7), and includes attachment means (12A, 12B, 12C, 19A, 19B) for holding said body member (7) in a fixed position on a chain cable (4) comprising a series of links (16, 17), whereby said longitudinal axis (14) is maintained substantially aligned with an axis (6) of said chain cable (4), said body member (7) being adapted to maintain said chain cable (4) extending in a defined attitude therein, said attachment means (12B, 12C) including first contact points (19A, 19B, 19C, 19D) on said body member (7) for transferring axial load therefrom to corresponding points on a link (17B, 17C) of said chain cable (4) and second contact points (A, B) on said body member (7) for transferring transverse load therefrom to corresponding points on a link (17A, 17C) of said chain cable (4).
 2. A fluked burying device (1, 1A), according to claim 1, wherein said defined attitude comprises alternate links (17) of said chain cable (4) being maintained in a plane at right angles to said plane of symmetry (5).
 3. A fluked burying device (1, 1A), according to claim 1, wherein said second contact points (A, B) are positioned such as to constrain induction of bending stresses in said link (17A, 17C) of said chain cable (4) during operation of said fluked burying device (1, 1A) to be similar to that which occurs in a wildcat of a windlass when tensioning said chain cable (4).
 4. A fluked burying device (1), as claimed in claim 1, wherein said second contact points (A, B) are positioned on said body member (7) at two locations to bear on two of said links (17A, 17C) spaced apart such as to oppose turning moment induced in said fluked burying device (1) during interaction with a seabed soil (38) while penetrating therein when said chain cable (4) is taut.
 5. A fluked burying device (1), according to claim 4, wherein said two locations of said second contact points (A, B) are spaced apart by not less than 12 times a bar diameter (D) of said links (16, 17) and, preferably, not less than 20 times said bar diameter (D).
 6. A fluked burying device (1, 1A), according to claim 1, wherein said second contact points (A, B) are positioned such that said corresponding load transfer points on said link (17A, 17C) lying in a plane at right angles to said plane of symmetry (5) are spaced from said plane of symmetry (5) by a distance in the range of 0.8 to 1.0 times a nominal of a bar diameter (D) of said link (17A, 17C).
 7. A fluked burying device (1, 1A), as claimed in claim 1, wherein said second contact points (A, B) are positioned such that said corresponding load transfer points on said link (17A, 17C) lying in a plane at right angles to said plane of symmetry (5) are separated from a central point of a crown section, lying in said plane of symmetry (5), of said link (17A, 17C) by a distance in the range of 0.4 to 0.6 times a nominal of a bar diameter (D) of said link (17A, 17C) measured in a direction parallel to said axis (6) of said chain cable.
 8. A fluked burying device (1, 1A), according claim 1, wherein said attachment means (12A, 12B, 12C, 19A, 19B) comprises a pocket (12A, 12B, 12C) in said body member arranged to accommodate a link (17A, 17B, 17C) of said chain cable (4).
 9. A fluked burying device (1, 1A), according to claim 8, wherein said pocket (12A, 12B, 12C) constrains said link (17A, 17B, 17C) of said chain cable (4) to lie in a plane at right angles to said plane of symmetry (5).
 10. A fluked burying device (1, 1A), as claimed in claim 1, wherein said fluked burying device (1, 1A) is formed substantially in two halves (2, 3) for opposed sideways assembly on said chain cable (4).
 11. A fluked burying device (1, 1A), as claimed in claim 1, wherein said first contact points (19A, 19B, 19C, 19D) are located adjacent said fluke member (8) whereby tensile loading impressed in said chain cable (4) by said fluked burying device (1, 1A) substantially bypasses that portion of said body member (7) lying forward of said first contact points (19A, 19B, 19C, 19D).
 12. A fluked burying device (1, 1A), as claimed in claim 1, wherein said body member (7) comprises two elongate members (9, 10) each disposed substantially parallel to said plane of symmetry (5) and each extending along one of two opposed sides of said chain cable (4).
 13. A fluked burying device (1, 1A), as claimed in claim 12, wherein said elongate members (9, 10) are plate-like.
 14. A fluked burying device (1, 1A), according to claim 12, wherein a pocket (12A, 12B, 12C) in said body member arranged to accommodate a link (17A, 17B, 17C) of said chain cable (4) comprises an elongate slot (12A, 12B, 12C) perforating each of said plate-like elongate members (9, 10) to accommodate said link (17A, 17B, 17C).
 15. A fluked burying device (1, 1A), as claimed in claim 12, wherein said plate-like elongate members (9, 10) are spaced apart by a distance between 1.06 and 1.1 times a nominal diameter of said link (16, 17).
 16. A fluked burying device (1, 1A), as claimed in claim 1, wherein said body member (7) has a width less than that of a link (16, 17) of said chain cable (4), measured transverse to said chain cable (4).
 17. A fluked burying device (1, 1A), as claimed in claim 13, wherein said plate-like elongate members (9, 10) each have a thickness (t9) less than a bar diameter (D) of a one of said link (16, 17) of said chain cable.
 18. A fluked burying device (1, 1A), according to claim 15, wherein said elongate members (9, 10) are splayed apart at an extremity (28, 29) such that a link (16A, 16B) of said chain cable (4) emergent from said extremity (28, 29) can swing freely sideways from said plane of symmetry (5) through an angle (⊖) of up to 20°.
 19. A fluked burying device (1, 1A), according to claim 18, wherein said emergent link (16A, 16B) can swing in said plane of symmetry (5) through an angle of up to 90° from said longitudinal axis (14).
 20. A fluked burying device (1), according to claim 1, wherein said fluked burying device (1) includes roll stabilizers (42).
 21. A fluked burying device (1A), according to claim 1, wherein said fluked burying device (1A) includes yaw stabilizers (46).
 22. A fluked burying device (1, 1A), according to claim 1, wherein said fluked burying device (1, 1A) is arranged such that a straight line (22) containing a centre point of a forward crown section, lying in said plane of symmetry (5), of a link (17A) in a foremost pocket (12A) in said body member (7) and a point of projection (25) onto said plane of symmetry (5) of a foremost point (20) of said fluke member (8) is inclined to said longitudinal axis (14) at an angle (δ) in the range of 25° to 35°. 